L to R: Rubidium metal, the first Rb BEC, and an electron shell diagram of Rb. Images from wikimedia and NIST.

Element: Rubidium (Rb)

Atomic Number: 37

Mass: two “stable” isotopes, 85 and 87 amu (rubidium-87 is technically radioactive, but it’s half-life is 48 billion years, so it might as well be stable for atomic physics purposes.

Laser cooling wavelength: 780 nm

Doppler cooling limit: 140 μK

Chemical classification: Alkali metal, column I of the periodic table. Like the majority of elements, it’s a greyish metal at room temperature. Like the other alkalis, it’s highly reactive, and bursts into flame on contact with water, even more so than sodium (in general, the alkalis get more violently reactive as you move down the column). For this reason, all physicists working with rubidium have True Lab Stories about accidentally blowing stuff up with it, and if you buy me a beer, I’ll tell you some.

Other properties of interest: Scattering length of around 100 a0; nearly identical scattering lengths for two trappable states in 87Rb; a whole slew of Feshbach resonances– this paper claims 40 in 87Rb; negative scattering length for 85Rb, but this can be made positive using a Feshbach resonance at around 230G.

History: Rubidium was a later arrival on the laser-cooling scene, but took off in popularity when it was realized that the diode lasers used for CD players worked in the near-infrared, at a wavelength very close to that needed to laser cool rubidium. That brought laser cooling within reach of a huge range of labs– rather than dropping a couple hundred thousand dollars on a dye laser system, you could spend maybe ten grand on the complete laser system, with money left over for vacuum hardware. When Eric Cornell and Carl Wieman got BEC in 1995, a lot of the press releases talked about how the whole apparatus only cost around $50K (which is a bit of a cheat, since that didn’t include the salaries of the many very smart people who put the stuff together, but those get left out of everybody’s calculations, so it wasn’t a major lie).

In addition to being enabled by cheap lasers, rubidium turns out to have a bunch of nice properties, particularly for people doing BEC experiments. It’s got hyperfine structure, meaning you need a repumping laser, but the frequency difference is relatively easy to manage, and that allows you to do “dark spot” traps to bump up the atom number. The number of atoms you can get in a condensate, and the energy of those atoms, depend on the collisional properties of the atoms in question, and those work out very nicely for rubidium– the “scattering length” that characterizes the collision for rubidium-87 is moderately large and positive, and by happy coincidence is nearly the same for two different magnetically trappable states, and between those states, so they’ve been able to do all sorts of fun two-species experiments. The collisional properties are also vastly better than those of cesium, so laser-cooled rubidium samples have found lots of applications in “clocks,” and for comparisons of time standards of different types.

Rubidium was the system for the first dilute-vapor BEC, earning Cornell and Wieman their share of the 2001 Nobel Prize. They’ve kind of gone back and forth with the Ketterle group at MIT ever since for “coolest recent BEC experiment,” with the JILA team being the first to see vortices in a condensate (one of the signatures of superfluid behavior), and all sorts of two-species stuff because there are two trappable states that will happily coexist. The Bose condensation of rubidium-85 was also pretty impressive, because 85Rb by itself has a negative scattering length which prevents the formation of a stable condensate, but they were able to change the collisional properties using a Feshbach resonance, opening the door for a lot of other experiments using those resonances.

These days the laser situation is less rosy than it was in the early 1990’s– the electronics inductry has moved on to different laser technologies, so the only people who still buy 780nm lasers are atomic physicists, and there aren’t enough of us to support a robust market for cheap lasers. But there are more commercial suppliers of complete laser systems these days, so it’s not all that bad. If you were going to set up a generic ultracold-atom system from scratch to allow a wide range of possible experiments, rubidium is still probably the atom of choice.

Random fun things: I’ve heard rubidium half-jokingly called “God’s atom,” a line that almost certainly originates with Eric Cornell. The properties are really that nice for doing BEC.

In college, I worked on making a rubidium MOT, and made a mix tape of the songs I listened to a lot that year which I labeled with the spectroscopic notation for the hyperfine transition for laser cooling in rubidium. That used to be good for some double takes whenever I’d give other physicists a ride someplace.

(It was a great tape, too, but I don’t seem to have transcribed the track list on the blog, alas… this one from the same era was also pretty awesome, though.)

Books

You've read the blog, now try the books:

Eureka: Discovering Your Inner Scientist will be published in December 2014 by Basic Books. "This fun, diverse, and accessible look at how science works will convert even the biggest science phobe." --Publishers Weekly (starred review) "In writing that is welcoming but not overly bouncy, persuasive in a careful way but also enticing, Orzel reveals the “process of looking at the world, figuring out how things work, testing that knowledge, and sharing it with others.”...With an easy hand, Orzel ties together card games with communicating in the laboratory; playing sports and learning how to test and refine; the details of some hard science—Rutherford’s gold foil, Cavendish’s lamps and magnets—and entertaining stories that disclose the process that leads from observation to colorful narrative." --Kirkus ReviewsGoogle+

How to Teach Relativity to Your Dog is published by Basic Books. "“Unlike quantum physics, which remains bizarre even to experts, much of relativity makes sense. Thus, Einstein’s special relativity merely states that the laws of physics and the speed of light are identical for all observers in smooth motion. This sounds trivial but leads to weird if delightfully comprehensible phenomena, provided someone like Orzel delivers a clear explanation of why.” --Kirkus Reviews "Bravo to both man and dog." The New York Times.

How to Teach Physics to Your Dog is published by Scribner. "It's hard to imagine a better way for the mathematically and scientifically challenged, in particular, to grasp basic quantum physics." -- Booklist "Chad Orzel's How to Teach Physics to Your Dog is an absolutely delightful book on many axes: first, its subject matter, quantum physics, is arguably the most mind-bending scientific subject we have; second, the device of the book -- a quantum physicist, Orzel, explains quantum physics to Emmy, his cheeky German shepherd -- is a hoot, and has the singular advantage of making the mind-bending a little less traumatic when the going gets tough (quantum physics has a certain irreducible complexity that precludes an easy understanding of its implications); finally, third, it is extremely well-written, combining a scientist's rigor and accuracy with a natural raconteur's storytelling skill." -- BoingBoing